Project Summary Adolescent idiopathic scoliosis (AIS) is the most common pediatric musculoskeletal disease, affecting 3% of children worldwide. The pathophysiology and molecular mechanisms of AIS remain poorly understood, however recent population studies have begun to identify associated risk loci, including GPR126. We have generated the first mouse model of AIS by knocking out Gpr126 in osteochondral progenitor cells, which give rise to both bone and cartilage tissues of the spine. Close examination of these mice uncovered defects in intervertebral discs (IVDs) and changes in the expression of well-established anabolic and catabolic factors important for IVD homeostasis prior to AIS onset. Recently, we find that loss of Gpr126 in osteoblast lineages does not generate scoliosis or bone defects, ruling out the role of Gpr126 in bone for AIS. In contrast, loss of Gpr126 in cartilage resulted in mild scoliosis in young mice that could progress in severity at later time point. Together these observations provide the foundation for the central hypothesis of this application: GPR126 has a critical role in cartilage for IVD homeostasis and spine stability. This hypothesis will be tested under two specific aims. (1) To determine the structural and mechanical changes of the IVD during the natural history of AIS and identify the molecular functions of Gpr126 for spine stability. We find no role for Gpr126 in bone development or homeostasis, however we do observe that loss of Gpr126 in cartilage resulted in IVD defects prior to and following the onset of scoliosis. It remains to be determined whether these defects are primary or secondary causes of scoliosis. We hypothesis that defects in the IVDs might be primary to scoliosis in our AIS mouse model. To test this hypothesis, we will determine the precise structural and mechanical changes of the IVDs as well as vetebra bodies during the natural history of AIS using contrast-enhanced 3D microCT imaging and mechanical testing. We will also comprehensively characterize cellular and molecular changes of the IVD using unbiased omics approaches, including RNA-seq, ChIP-seq and mass spectrometry performed on primary IVDs isolated prior to and following the onset of AIS. (2) To determine whether inflammation is a major driver of AIS and IVD degeneration following loss of Gpr126. Our preliminary results show that loss of Gpr126 in cartilage leads to severe IVD defects and abnormally activated IL-6/Stat3 signaling prior and after the IVD degeneration. To test whether Gpr126 interacts with the IL-6/Stat3 signaling to modulate cartilage development and homeostasis, we will utilize our cartilage-specific Gpr126 knockout mouse and genetically test if loss of Stat3 will ameliorate disc degeneration and scoliosis. We will complement this genetic approach using a pharmacological approach to administer Stat3 inhibitors or nonsteroidal anti-inflammatory drugs (NSAIDs) to determine whether blocking these inflammatory pathways can alternate IL6/Stat3 activation in cell culture, and ultimately determine if we can alleviate AIS progression and disc degeneration in vivo.